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Starting 6 kV , 1.35 MW Syncronous Motor
6

Starting 6 kV , 1.35 MW Syncronous Motor

Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
In one excavator there is 6 kV synchronous motor starting direct on line. Motor is starting with small load but with high inertia. Motor is running 3 1MW DC generators (Vard - Leonhard group). Starting time is almost 1 minute. Because of direct on line start starting current is too high and circuit breaker that supply this excavator trips. We must to solve this problem but with low budget. Idea is to use small AC motor with frequency converter to run this Vard Leonhard group up to synchronous speed and then to close motor circuit breaker but with no exitation on motor. After this we will increase exitation on motor up to nominal value.
I am interesting what is your oppinion on this and which starting current should i expect with starting like this.
Also another question is how much should be nominal power of this small motor.
Sorry for my bad english.
  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Difficult to say for sure, but as an example a pony motor arrangement used with a four pole 3.3kV 2000HP induction motor was about 90kW if I remember correctly. If this is a two pole motor then I expect the power required would be higher.

Your English is quite clear, so no need to apologise. In England we know the motor-generator system as "Ward Leonard" sets which might help if you are ever looking for information on them.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
ScottyUK,

Motor is 6 pole , 50 Hz , 1000 rpm. Problem is that this pony motor should run this synchronous motor and 3 more DC generator 1 MW , 1000 rpm which are mechanically coupled on same shaft.
I have some information that on the same configuration somebody used 55 kW pony motor but this information is not 100% sure.
But what concern me more is how much will be current peak after I close main breaker of synchronous motor.
Anyway thanks for suggestion.
 

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Milovan, I would rather concentrate on Circuit Breaker trip settings. Your synchronous motor (SM) already has asynchronous motor (AM) inside it, much larger size then you are considering for your pony motor. SM can not start on its own, it must be brought to near nominal speed by other means. For that purpose, there is a cage added on the rotor, so SM can start as AM and then switch to SM mode. Control logic is a problem not the motor.
  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

2
What type of bearings do your generators have. If you have babbit bearings with oil rings you may find that turning the set by hand for several minutes to bring up oil may reduce your starting times and avoid tripping.
Years ago, we started every shift by manually rolling 3000Hp synchronous motors on an excavator. It took a minimum crew of four but went better with six men. After test and commissioning the machine ran 24/7.
You may be able to use the synchronous motor with a VFD instead of a pony motor. Some of the folk here have experience with this technique. Be patient.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Starting 6 kV , 1.35 MW Syncronous Motor

2
Can't you not use one of these DC generators as a motor for start up ?

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Wouldn't a reduced voltage starter be less expensive than another motor and VFD?

RE: Starting 6 kV , 1.35 MW Syncronous Motor

You are just accelerating the rotors of 3 DC generators. It is just an inertia load and even then shouldn't be a huge load for the motor. It doesn't seem right that this takes 50 seconds.

You would have to figure out how much HP is required just to spin everything at 1000rpm. Then you could size this pony motor.

The current peak when you close the main breaker would be the same as you see now, it would just be a much shorter time before the running current levels are reached. However, you could apply the field to the motor first and then synchronize the motor to the line power before closing the breaker to reduce the inrush.

If there was a bearing issue like Bill described, I would use a low voltage VFD connected to the motor before starting. You could run the motor at full torque at 3.2Hz using a 380V VFD by setting the proper V/Hz in the VFD. This would just require a VFD system that can handle the motor FLA which isn't that much.
 

RE: Starting 6 kV , 1.35 MW Syncronous Motor

The best bang for the bucks will be by running one of those DC generators through a DC drive. Best starting torque - smooth and with minimum current & fuss. Ramp down and disconnect the dc drive once the system is running up to speed with the synch motor.

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
"Your synchronous motor (SM) already has asynchronous motor (AM) inside it, much larger size then you are considering for your pony motor."


Tommy007 I cannot agree with you. This thing about cage added on the rotor is correct and SM can start direct online becacause of this cage. But current with starting like this is about 5-6 nominal current of SM. So this mean that in starting you have about 7.5 MVA and this is much more higher then 50-100 kW of pony motor.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Warros,

This with bearings is very interesting but we dont have so much people on excavator. There is only one man who is working on excavator per shift. But with more effective bearings we can reduce power of motor and VFD. Thaks for suggestion.
I dont believe that any solution on 6kV side can be cheaper than pony motor and VFD on 0.4KV.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
djs,

I dont believe that any solution on 6kV side can be cheaper than pony motor and VFD on 0.4KV.  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
LionelHutz,

It is time like this believe me. This is hugh inertia because weight of this 4 machine is about 50t and also you have some friction losses.

About this that you wrote "The current peak when you close the main breaker would be the same" I thought like you just I am not 100% sure. If this is correct than we must see which protection is tripping circuit breaker. If this is some overload then this solution with pony motor is OK and if protection that trip CB is some kind of short circuit protection than we are in the problem because breaker will trip again wherever speed is synchornous or zero when you close CB.

And this what you wrote about firs exitation and syncronization motor on network before closing CB is best techincal solution and I was thinking abuout this just I am not sure how much money this will cost. But I agree with starting like this current will be very low.

Thanks.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

I agree with you about using DC converter on one of DC generators. I was thinking about this also.
Only few things to explain.

Nominal ratings of DC generators are 900VDC, 1000kW, 1000rpm. We have 400 VAC available. OK we can reduce exitation on this DC generator and run this machines up to synchronous speed of SM like this. So when you rectify 400 VAC you have about 540 VDC so this mean that flux should be about 60% of nominal and stator current about 40-50% of nominal.

For this solution I need 2 extra more contactors for switching generator between drive and 2 motors (which this generator supply). This contactors should be for 900 VDC (and this is not standard) and one should be for 1000kW and another for about 70-80kW.

So when you calculate all of this price is more or less same. But this version of solution is still active.

But I am afraid what will be with starting current of SM when I close CB.

Thanks.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Milovan

Yes, you could achieve the final speed with field weakening of the dc motor (generator). Or a step-up 400/700 V trafo feeding the DC drive.

The set-up I see is

For the field circuit - Use the existing one.

For the armature - A parallel feed via 400/700 V AC; 520/900 V, (100 KW ?) DC drive with a 1000 V rated motorized isolator (instead of a contactor)at the drive output.

If you want a smooth synch of your motor with minimum inrush, you could also run your synch motor as a generator with external excitation and then synch with a synchroscope & synch check relay. But this again is a cost driver with associated maintenance/operation issues.

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

I prefere to do this without step up transformer (because DC drive for 900VDC is also not standard).

This motorized isolator then should have 3 pairs of contacts to connect: drive, generators and motors and should be for 1000 kW. Is that what you mean.

Do you maybe now how much cost some synch reley that I can use for synchronization.

Thanks.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Milovan

The problem with field weakening is it also weakens the dc motor torque and this could result in acceleration issues for you. Hence, it is better to resort to field weakening only as a last option.

I agree 700 V DC seems to be limit in the DC drives (at least per Allen Bradley). Still it is better than 520 V DC in terms of better acceleration with a stronger field.

My idea is to run a parallel DC supply to only one of the DC generators from the DC drive with a 1 KV rated motorized isolator. I'm rating this isolator at 1 KV since there will be a 900 V voltage backfeed from the DC generator when it is running normally.

The whole system will have a rating of about 100 KW (?) - you need to do the math for accelerating the 50 ton mass from 0 to 1000 RPM.
 

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

All of these discussions are good but the first order of business is to determine if it is a problem for the motor due to long acceleration time.
If it is OK thermally and OK for the drive bearings, then adjust the CB not to trip on acceleration.
JIM

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Lansford,

You are right, but:

- Another thing that is problem with starting like this is voltage drops that occur on other devices on the same 35 kV /6 kV trafo station.

- and one more important thing is that I am working in company that would like to solve this problem and also to earn some money. But profit will be very low if we solve this problem by only increasing settings on protection.

Thanks.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Milovan - I see what you mean (BTW, I think you have 2 dc motors connected with the synch motor wrongly).

What you need is a two pole isolator-cum-changeover switch or contactor wherein in one position, you connect the external dc drive supply to the generator armature (to run it as a motor) and in the other position, you connect the DC motors to the generator armature. All of them must be rated for 1 KV and I suggest to make all changeovers at zero or low armature voltage to reduce arc flash.

Am I boring you too much ? :)

 

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

No , No (I am think on boring).

Now its OK about three position switch disconnector, but with 1000 kW rating (not because of drive, because of motors).

This thing with changeover on low voltage I agree with you especailly because current is DC.

And what about pony motor. You prefere more this solution with dc generator?

Thanks, bye.
  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Are the DC motors hard-wired directly across the generator terminals without an intervening breaker ? If yes, how do you protect the motors from faults ?

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
"Are the DC motors hard-wired directly across the generator terminals without an intervening breaker ? If yes, how do you protect the motors from faults ?  "

Edison123,

As I now connection is direct. This is some old Russian solution and they have some current measurement in DC circuit (because of speed regulation on dc motors) and I assume that they use this measurement also for protection. So if they want to switch off dc motors they simply reduce exitation on DC generator to zero.
But I am not 100% sure on this.

Sorry if my english is bad.  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison,

One more thing about protection from fault. If you have some short circuit fault in DC circuit, I now that decreasing exitation on dc generator is too slow.
But than this cause also increasing current in 6 kV AC side and you have circuit breaker here.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

I looked now some schems that I have and it seem to be that there is no CBs in DC circuit.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

It's possible they have wired to trip the generator field in case of any fault. My suggestion would be to introduce a two pole contactors/isolators for both the motor (100 KW rating) and the external dc drive supply (100 kw ? rating) and have an interlock mechanism to switch only one of them at any given time.

As for pony motor, does your synch motor or even one of the generators have a shaft extension in the opposite end to fit a pony motor ? Then you need a VFD to power up this motor and a VFD will be a costlier option to buy and maintain than a dc drive ? And then there is the additional cost of purchase, erection and maintenance of the pony motor.

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

Yes there is possibility to install pony motor on shaft. I agree that this will be a little cheaper solution (but realy a LITTLE cheaper because we can buy 55 kW motor for 3200 euros and this contactors or isolators are not cheap).


 

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Then it's time to work out the comparative costing. Good luck with your project.

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123 thanks.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

correction

"My suggestion would be to introduce a two pole contactors/isolators for both the motor (1000 KW rating) and the .... "

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

I notice that this was accidentally mistake.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Yes, as my age goes up, my intelligence seems to go down. :)

Thanks for an interesting discussion. Come back here when you solve it and update the story.

 

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

OK I will.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

In regards to the bearing issue;
If you have babbit bearings the weight of the rotors will squeeze the oil out of the bearings when the machine is sitting. Typically the machine is turned slowly until the oil rings carry sufficient oil into the bearings from the oil sump.
The torque required to turn the rotors when dry is much greater than the torque required to turn them when the oil is flowing. Possibly more than ten times the torque is required to start the rotors turning than is required when they are running free.
There are several options;
DOL starting will create issues with both extended starting times and bearing wear.
If the issue is bearings, you may use a small pony motor to rotate the rotors slowly until the bearings are properly oiled and the turning torque drops. Then start DOL.
In regards to weight, mass inertia;
Our sets weighed 80 tons to your 50 tonnes. BUT you don't accelerate the whole mass. You only accelerate the rotors which is a small part of the total mass.

NO breakers;
This is a common Ward Leonard scheme. Our sets used loops of two generators and two motors in series for each loop. Protection is by generator field reduction and stopping the main synchronous motor. The possibility of  expensive mechanical damage to the machine is greater than the possible electrical damage if a drive loop goes open with an elevated load on the machine.

Using a DC motor for a pony motor;
Use full field. Start with zero voltage on the armature and ramp the voltage/current up. Your current will follow ohms law. R will be the resistance of the armature. E will be the applied voltage minus back EMF.

An anecdote to illustrate motoring DC generators.
Many years ago I was serving as a replacement instructor at a vocational school.
In our shop there was an old DC generator that had been donated to the school. As I remember it was about 250 kW or 300 kW. In class we had been discussing DC machines and went out to look at the old generator. I decided to see if we could motor the old generator. We had power supplies available that were designed for electronic experiments. The output was about 3 amps. We used the largest supply to energize the field and connected a variable supply to the armature. We set the current at about 2 amps and the generator started to turn very slowly. As the rotor turned the current dropped. As the current dropped, we set the voltage a little higher. The old machine eventually reached a speed where the floor was starting to bounce.
I was about to terminate the experiment and let the achine coast down when one of my students tripped over the extension cord to the power supplies. The inductive kick let the smoke out of both power supplies.

Suggestion, It may take much less energy than you expect to spin unloaded rotors.

Suggestion, Lift the brushes on a couple of motors to break the loop. Connect a welding machine to the generator armature (with the field applied) and monitor the current and voltage to determine the capacity you will need for the permanent drive.
With full field applied to the generator the speed will be proportional to the applied voltage.
Before going further you should determine what type of bearings are installed on the generators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Waross,

Thank you very much for this detailed desctiption. Also that ancdote is very nice.

I dont believe that there is babbit bearing but I will check that if I could, because this machine is not in my country and my company only gives offer for this job.

About full field. I know its better to have full field because of smaller armature current. But then I have problem to find DC drive with 900 VDC output (we commonly use Siemens or ABB drives and they dont have drives for such high voltage).

Because you have so much experience I would like to hear what is your opinion on using serial connected two DC motors on one DC drive.

Thanks.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Milovan

I meant to ask you this. How a 1.35 MW synch motor is able to feed 3 MW DC load ? The power balance doesn't compute.

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

May I answer that, Muthu? The loading on the various machine functions is such that the generators are almost never loaded to the maximum simultaneously. That is, if the digging motors are loaded to the maximum, you won't be swinging. If this is a dragline type of excavator, the drag and hoist winches oppose each other. When the hoist is winding in, the the drag is paying out with very little load back to the sync motor.
The machine I was on had 5800 Hp total of motors on each 3000 Hp sync motor. There were four such sets.

I will think for awhile before answering the question Re two DC motors on one drive.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
Edison123,

Generators are working at same time but laod is very variety with time and maximum load on 3 generators are not in the same moments so 1.35 MW is enough for this 3 genrators.  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
warros,

Yes it is dragline excavator type Esh 10/70. This is russian name but I dont know if you know it according to that name or some other american name.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
warros,

When I asked you about this 2 DC motor, I concretly thing on 2 motors mechanicaly coupled and of course with same nominal ratings. Just to be more precize.  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Thanks Bill. Got it.

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

I assume that two motors are used to allow a reduction in the size of the ring gear or crown gear. If only one motor was used, the gear on the winch would have to be at least twice as strong. Two motors will also give some runaway protection should one motor fail. I am assuming that there are two motors on one end of the winch drum. If the motors are at opposite ends of the winch drum some of the same reasons still apply. Two motors will also balance the drive torque somewhat. Electrically I don't see a problem. I believe that this is a mechanical issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Back to your starting issues. Is there more than one excavator on-site? There is a very old technique for DOL starting of excavator motors on a weak system.
All the other excavators raise their buckets as high as possible. Then (co-ordinated by two way radio) they all drop their buckets in unison. As the buckets fall they regenerate. While the other machines are regenerating the machine coming in is started. How do you start the first one? With no machines running, the grid will often support one starting. Or one or two machines may have reduced current starting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Starting 6 kV , 1.35 MW Syncronous Motor

(OP)
warros,

thanks.

This is fantastic idea for starting motor.

I dont know detail about starting of excavators. I find some cheap sync reley so I will probably use it for synchronizing exitated SM on network.  

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Love that, Bill! PLS for you.

Even if it cannot be used in this case, it still is a great and fascinating idea.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Hmm, I know we'd have little problem building an fairly economical phase-controlled rectifier capable of 900VDC but it wouldn't be a DC drive. You could current limit or ramp the voltage but that'd be about the extent of the control.

RE: Starting 6 kV , 1.35 MW Syncronous Motor

I bet all those machines have lots of tales to tell about you, Bill. :)

Muthu
www.edison.co.in

RE: Starting 6 kV , 1.35 MW Syncronous Motor

Hello Muthu
One of the memorable service calls was first by dugout canoe and then by horse back to look at a small generator in the Moskito Coast region of Central America. Regrettably I was unable to repair the old beast. A shortage of resources, I was unable to find a battery suitable for flashing the field.
I can only do so much with a multi meter and no power of any kind. I have also worked on larger installations.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

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